Thermionic discharge tube with electronic velocity filter



Get. 5, 1948. A. LEVIALDI 2,450,592

THERMIONIC DISCHARGE TUBE WITH ELECTRONIC VELOCITY FILTER ATTORNEYPatented Oct. 5, 1948 THERMIONIC DISCHARGE TUBE WITH ELECTRONIC VELOCITYFILTER Andres Levialdi, Buenos Aires, Argentina, assignor to HartfordNational Bank and Trust Company, Hartford, Conn., as trustee ApplicationAugust 17, 1944, Serial No. 549,889

11 Claims.

This invention relates to velocity filters for electron beam and moreparticularly to thermionic discharge tubes utilizing monokinetic beamsof low-velocity electrons.

It is already known to filter high-velocity electron beams in whichhigh-intensity electromagnetic fields may be easily utilized forseparating the electrons into substantially monokinetic beams.

However, in electronic devices or thermionic discharge tubes whichutilize accelerating potentials approximately equal to the mean initialvelocity of the generated electrons, i. e. in devices operating withaccelerating potentials of the order of a few volts, filtering of theelectrons constitutes a rather difiicult problem for which, so far as Iam aware, no satisfactory solution has been found as yet.

One of the main difficulties encountered in filtering low velocityelectrons consists in that the intensity of the electromagnetic fieldswhich could be used for filtering purposes decreases to values fallingwithin the range of the intensity of the terrestrial field duringmagnetic storms, so that even with special shielding materials anddevices it is almost impossible to maintain the path-controllingmagnetic field within the predetermined values.

However, I have now found that low-velocity electrons possess someinherent characteristics which may be used advantageously for filteringpurposes. In fact, low-velocity electrons have relatively largeassociated wave lengths varying from 5.5 A. for accelerating potentialsof the order of volts to 17.3 A. for accelerating potentials of theorder of 0.5 volts in accordance wit the Broglie wave length formula.

Hence, even for electrons moving with nearly like velocities theassociated wave lengths differ by considerable values, so that bydirecting the electron beam toward an adequate difiracting latticestructure, the electrons of the diffracted beam will be distributedaccording to the velocities thereof, thus making it possible to select abundle of electrons having like velocities by means of an apertureddiaphragm located in the path of the difiractedelectron beam.Consequently, the electrons pas-sing through the diaphragm aperture canbe regarded as a monokinetic electron beam containing electrons ofsubstantially like velocities determined by the glancing angle of theincident electron beam, the grating distance of the diffracting latticestructure, and the position of the diaphragm aperture within the area ofthe diffracted electrons, the velocity range of the monokinetic electronbeam being further determined by the dimensions of the aperture.

It is therefore one of the main objects of the present invention toprovide, in thermionic discharge tubes, a simple and efiective means forobtaining a monokinetic beam of low-velocity electrons.

A further object of the present invention is to provide, in thermionicdischarge tubes, a velocity filter for low-velocity electrons, thefiltering characteristics of which can be varied over a continuous rangeof electron velocities.

These and other objects and advantages of the invention will be apparentfrom a consideration of the following detailed specification taken inconnection with the drawings which form part of the specification, andwherein:

Fig. 1 is a schematic longitudinal section of a thermionic dischargetube incorporating an electron-velocity filter according to myinvention;

Fig. 2 illustrates a modification of the electronvelocity filter shownin Fig. 1;

Fig. 3 shows another modification of the electron-velocity filterallowing of a coaxial mounting of the discharge tube elements;

Fig. 4 is a cross section of the thermionic discharge tube taken atarrows 4-4 of Fig. 3;

Fig. 5 shows another modification of the electron-velocity filterillustrated in Fig. 2; and 1 Fig. 6 illustrates a thermionic dischargetube similar to that shown in Fig. 4 but utilizing the electron-velocityfilter of Fig. 5.

The same reference characters or numbers in-' dicate like orcorresponding parts or elements throughout the drawings.

Referring to Fig. 1, there is shown a thermionic discharge tubecomprising an evacuated L-shaped tubular envelope If! the shorter leg ofwhich includes an electron gun H connected by means of terminals l2 andI3 to an adequate power supply source not shown in the drawings. Theelectron beam, emerging from an electron gun II at the orifice I4, isaccelerated and shaped into a convergent beam [5 by means of anelectrostatic lens system It constituted by a first and second anode l6and I 6", respectively, each connected to an intermediate potential of adirect current supply I1, the negative pole 11' of which is connected toterminal 13 of electron gun H, while anode I6" is also connected toground. The lowvelocity electron beam I5 is focussed on a point l8 andpenetrates into a diffraction zone l9 limited by the second anode l6" ofelectron lens system I6 and a diaphragm 20 also connected to 3 groundand provided with an aperture 2| so that substantially all points ofdiffraction zone I9 are at the same potential.

The convergent electron beam 55 is directed toward a diffraction element22 of regular lattice structure located midway between orifice I 4 andfocus [8 and having a substantially plane diffracting surface 22 placedat an angle of 45 with respect to the direction of the incident electronbeam I 5, so that a glancing angle 01 of 45 is formed betweendiffraction surface iig' and the incident electrons. The diffractionelement 22' can be formed by a nickel or quartz plate, the choice of thematerial depending' 'upon the lattice structure desired. Preferably, thereflecting The monokinetic and focussed electron beam surface of theplate is cut with certain crystal- I lographic orientation angles'with'respect to the crystallographic axes of the material.

It has been shown by de Broglie that electrons moving at a velocity muchbelow that of light with kinetic energy equal to V electron-volts, haveassociated a wave length which is given in angstroms by the formula Now,since diffraction element 22, which may be formed of a crystal or metalhaving an adequate lattice structure, may be regardedas a plane gratmgof grating space at equalto the distance between the atom rows of thelattice structure, the incident electron beam i willbe diffracted inaccordance with de Braggs formula n; \=d sin 01 where x is the wavelength; 61 the glancing-angle of the incident electron beam I5, (I thegrating space of the lattice structure and 12' any positive integer.With a fixed 'angle of incidence, the electrons of the diffracted beam15 will be dispersed over a relatively wide area covering a definiterange of anglesaz, formed between, the diffracted electrons of beam l5and the plane diffracting surface 22'. Foreach angle 02 the diffractedelectrons will have an associated wave length in accordance with'ideBraggs-fornrula. Hence, in the diffracted-beam t5 the electrons will bedistributed in-accordancewitlr their velocity, electrons of lowervelocity corresponding to larger angles 02' and vice versa. Thedistribution of the electrons within the dispersed diffracted electronbeam i5 will depend naturally upon the distribution of the mean electronvelocities present in the incident beam l5, so that the maximum andminimum values of 02 are determined by the minimum and maximum velocityof the electrons emerging from orifice I4 of gun I I. However, althoughscattered, the electrons of diffracted beam 15" will be focussed on aline which, in first approximation, can be represented by a circle 23drawn with the distance between plane diffracting surface 22 and focusl8 as 'a radius; and aperture 2| of diaphragm'zll is located to coincideprecisely with a point on the circumference of circle 23 which forms aminimum angle 0'2 withdiffracting plane 22, so that the electronsemerging fromdiaphragm aperture 2} will forma monochromatic and focussedelectron beam l5" containing electrons o f very like velocities, sincethe'diameter of aperture 2| can be made as small as desired.

Itwill be understood that, in the position shown of aperture 2 i Lelectron gun i I together with diffraction element 22 and diaphragm 20,consti- 15' emerging'from diaphragm aperture 2| should be regarded asatool prepared for further electronic work in devices requiring apractically perfect definition of electron velocity. In the ther-:mionic discharge tube shown in Fig. l the monokine'tic electron'beaml5" traverses modulation means indicated with the general referencenumeral 24 in which the electron beam can be transformed or modulated inaccordance with theparticular performance characteristics of thethermionicdisgharge tube in response to a modututes an electron velocityfilter having a predetermiffed arid'fi'xed filteringcharacteristic,since the mean velocity of the electrons in monokinetic lation potentialapplied to terminals 25 and 26 connected to modulation means 24.electrode 21, c onnected to the positive pole I1" of D.- C supply l 'lthrough a load impedance 23, may be formed of a fluorescent screen only,or may be constituted by an apertured target electrode, while loadimpedance 28 may be a pure; resistance,- a reso'na nt circuit, or a mereconductor the case of a thermionic discharge tube utilizing afluorescent screen as indicating means,

In the thermionic discharge tube of Fig. l, the incident electron beam[5 is focussed by means of an electrostatic lens system 16, the electronvelocity of monokinetic beam [5" being determined by the position ofdiaphragm aperture 2|. The thermionic discharge tube shown in thedrawing of Fig. 2 di ffe s from that of Fig. 1 in that the incidentandthe diffracted electrons are travelling in divergentbe arns and thatfocussing of tl fe -electrons is applied to the 'monokinetic beamemerging from diaphragm aperture 2 I. For

' purpose; a lens system, designated with general referericenumeral 29and connected to an intermediate potential of D.-C. supply I1, is 1ocated between diaphragm 20 and modulation means 2 'l'of the tube, thediffraction zone 19 of which is limitedhyan accelerating anode 38 andthe diaphragm 25.3, both connected to ground potential. Furthermore,diffraction zone id includes a pair of deflector plates 3! and 32connected to control terminals 33 and 34, respectively, so that byapplying an adequate potential to these terminald theangle of incidenceof electron beam 55 on diffracting surface 22 may be varied withincertain limits, as shown by the dotted lines35 and 35. Consequently, thevelocity of theelectronsin the monokinetic electron beam (5" will varyin accordance with the control potential applied to control terminals 33and 34, so that a monokineti'c electron beam of adjustable mean velocitycan be obtained.

Although the electronic performance of thermionic discharge tubes shownin Figs. 1 and 2 is quite satisfactory, the L-shaped envelope i0constitutes a constructive disadvantage which, under some circumstances,may restrict the utilization of the thermionic discharge tubes includingthe electron monochromator or velocity filter according to thisinvention.

Figs. 3 and 4 of the drawings show a longitudinal and cross section of athermionic discharge tube, respectively, comprising a tubular envelopel0,

Collector wherein the difierent construction elements of the tube can beconnected to the contact pins of a standard base, if desired. As can beseen in the drawing, electron gun I, electrostatic lens system l6,diaphragm 26, modulating means 24 and collector electrode or target 29are coaxially mounted on a virtual axis parallel to that of tubularenvelope I and coinciding with the axis of gun l I. Diffraction zone orchamber |9 is limited, similar to that of Fig. 1, by the second anodeiii of electrostatic lens system it and diaphragm 20 and includesdiffraction element 22 provided with a plane diifracting surface 22.However, difiraction element 22 is not located in the direct path of theconvergent electron beam l emerging from orifice M of lectron gun H, butis placed in a plane parallel to the axis of envelope Hi, and convergentbeam l5 is deflected toward diffracting surface 22' by means of amagnetic field perpendicular to the plane of the drawing and generatedby a pair of coils connected in series to control terminals 33 and 34'to which an adequate D.-C. supply source (not shown in the drawing) isconnected. The controlling magnetic field, formed as the sum of theindividual magnetic fields schematically indicated with arrows 38 in thedrawing of Fig. 4, also deflects the diffracted electrons towardsdiaphragm 20 and the difiracted electron beam I5 will be dispersed overa relatively wide area in a manner similar to that shown in Fig. -1.Hence, the electrons emerging from aperture 2| of diaphragm 20 form amonokinetic beam including electrons the mean velocity of which isdetermined by the position of the diaphragm aperture 2| within the areaof the diffracted beam l5. However, by varying the amplitude of themagnetizing D.-C. voltage applied to control terminals 33? and 34', i.e. by varying the intensity of the electromagnetic field generated bycoils 37, the glancing angle 01 of the incident electron beam l5 may bevaried within certain limits, so that an adjustable velocity monokineticelectron beam will be obtained.

The position of diaphragm 20 in the thermionic discharge tube shown inFigs. 3 and 4 is elected so that the sum of the mean trajectories of theincident and difiracted electron beams 5 and l5, respectively, issubstantially equal to the focal distance of electrostatic lens systemit, so that monokinetic electron beam l5" emerging from diaphragmaperture 2| is focussed to obtain a maximum concentration of theelectrons.

In the thermionic discharge tubes shown in Figs. 1 to 4, the focussingof either the incident or the monokinetic electron beam is obtained bymeans of suitable electron lens systems. The focussing of the electronbeam can also be obtained by utilizing a concave diffraction elementmounted in a manner similar to that used by Rowland for his spectroscope(A. C. Hardy and F. H. Perrin, The Principles of Optics, 1932, pg. 563).As can be seen in the drawings,- the thermionic discharge tube shown inFig. 5 is similar to that disclosed in Fig. 2, with the only differencethat a diffraction element 39 having a, concave diffracting surface 39is utilized in the difiraction zone It" which is also provided withdeflector plates 3| and 32 for varying the glancing angle of incidentelectron beam IS. The curvature of concave surface 39' corresponds to acircle 40 passing through orifice Id of electron gun H and diffracted asa plurality of convergent monokinetic beams focussed on different pointslocated on the circumference of circle 40, as indicated by beams I51 andl52 in the drawing of Fig. 5.

The aperture 2| of diaphragm 20, being located at the focus of electronbeam l5'1 will thus constitute a source of a concentrated monokinetic Ielectron beam I5, containing electrons the velocity of which isdetermined by the position of diaphragm aperture 2| with respect todifiracting element 39 and orifice M of electron gun II. 'By varying theglancing angle of incident electron beam Hi, the velocity of themonokinetic electron beam l5" can be also adjusted within certainlimits, as already explained hereinbefore with reference to the electronvelocity filter incorporated in the thermionic discharge tube shown inFig. 2. In the thermionic discharge tube shown in. Fig. 6 the concavedifiracting element 39 is mounted in a way similar to that of planedifiraction element 22 in the tube disclosed in Fig. 3, so that theelectron velocity filter including concave difira-cting element 39 canalso be used in a thermionic tube comprising a tubular envelope ill, thediffraction zone |9" of the tube being limited by an accelerating anode3i] and the apertured diaphragm 20 both connected to ground. Thedeflection of the incident and diffracte-d electron beams l5 and I5,respectively, is obtained by means of an electromagnetic field generatedby coils 31, as already explained hereinbefore with reference to Fig. 3.

While I have indicated and described several embodiments of thermionicdischarge tubes incorporating electron velocity filters in accordancewith my invention, it will be apparent to one skilled in the art thatmany modifications may be made without departing from the scope of myinvention, as set forth in the appended claims.

I claim:

1. A thermionic discharge tube comprising an evacuated envelopecontaining means for generating a. beam of electrons, electrode meansfor l r tin a directing said electron beam to a diffraction elementhaving a substantially regular lattice structure to produce a difiractedbeam of electrons dispersed in accordance with the respective velocitiesthereof. a diaphragm provided with an aperture and located in the pathof the diffracted electron beam to produce an emergent beam ofsubstantially monokinetic electrons the velocity of which is determinedby the glancing angles of the incident beam and said monokineticelectrons with respect to the difiracting surface of said diffractionelement, said accelerating elec trode and said apertured diaphragm beingelectrically interconnected to form a substantially equipotential spaceincluding said diffraction ele ment, means to vary the initial velocityof said accelerated electron beam to vary the velocity of said emergentmonokinetic electron beam, and a target electrode to collect the saidmonokinetic electron beam.

2. A thermionic discharge tube including an evacuated envelopecontaining means for producing, accelerating and focusing a beam ofelectrons to a diifracti'on element, said element having a latticestructure arranged so as to disperse the said electrons with respectivevelocities approximately as a function of the glancing angles of theincident beam, electron barrier means provided with an openingpositioned in the path of the said diffracted beam so as to passmonokinetic electrons of approximate like velocity, said bar- 7 riermean's and sald accelerating means being electrically interconnected soas to. include. the said 'd'ifiralc'tion element: in a'substantiallyequipotential space, means to vary the initial velocity of the saidbeam-therebyto vary the velocity of said passed electrons, means tomodulate th'esaid passed ele'ctrons in accordance with electric controlenergy; and atargct electrode to collect the said modulated pass-edelectrons.

3. A. thermionic discharge' 'tube comprising-an evacuated envelope andwithin'the envelope an" electron diffraction element; means forproducing, accelerating, and focusingabeam of electrons to-' ward's-ai'd diffractlon element, said element having a lattice' structurearranged"- to disperse the said electrons in divergent directionsproportional to the respective velocities thereof;"electron barriermean's provi'ded with an opening positioned in the path of the saiddispersed electrons to pass a monokinet'ic beam of-electrons-of'sub'stantially like velocity, :s'aid barrier means-and saidaccelcrating means being electrically interconnected J" to provide asubstantially e'quipotential space 'enclosed said diffracticn element;target elec-" trodetd collec't the said "passed electron beam, andmeans-intermediate said opening an-d said target electrode to modulatethesaid passed 'elec-s' tronbeam in accordance with'electricalcontrolenergy. i 4. A thermionic-discharge tube' comprising an,evacuatedxenvelope' and withincthe envelopeanelectron 'diiTra'cti-onelement, means tolgenerate a beam of electrons and to rocusfiand'directsaid electron beam toward said-diffraction "element;

said element having a lattice "structure arranged to disperse thesaid-electrons indivergent directions proportional tothe velocities'thereofi 'a 'diaphragm" provided withan aperture positioned in thep'athliof 'the said dispersed electrons to pass a monokineti'c' beam of"electrons of substantially like velocity, said diaphragm and saidfocusing I.

and directing means being electrically interconheated-and enclosingth-e'said 'diffractiorrtelement-i in a substantially 'equipotential"space, a targeti electrode-tot collect the said passed electron beamsaid generat ng; focusing, "and .directingmeans: being adaptedtovaryth'e velocity of theelectrons; of the passed electron'bearrr, andmeans-,iintermen di'ate r said aperture and said :tar'getelectrodei'tomodulate said passed electron beam;

5. A thermionicdischarge tube comprisingan" evacuated-envelope andwithin'the envelope an electron diffraction element; means -for'generata beam-oi: electronsymeans to-accelerate and direct said electronbeam-toward said (infraction:- elemenizsaid element having'a'substantia'lly re'gular lattice structure arranged to disperse-by'diffraction the said electrons in divergentdirections according-"to:the respective" velocities thereof, electron barrier means provided withanropening positione'diin the path of the said dispersedelectrons topass amonokinetic'bea'rn of'electron's of substantially like velocity;said barrier'meansand said accelerating and directing means beingeleccollect' the said passed electron beam,and means intermediate saidopening and said target 'elec-' trode to mo'dulate said passed electronbeam.

6. A thermionic discharge tube comprising an static'cb'eam deflecting'means'tovary the angle 7 of incidence of said generated electron beamon said difiracting surface in accordance With'ele-ctnicalicontrolenergy, a target electrode to collect the said passedelectronbeam, andmeans intermedia'te'said openin and said target electrode tomodulate-said passed electron beam.

7. A' thermionic discharge tube comprising an evacuated envelope'andwithin said envelope an electron diffraction element, means forgenerat-' ing a beam of electrons, means to'accelerate and direct saidelectron beam toward said diffraction clement, said element having asubstantially reg- 1lular -lattice' structure arranged to disperse bydilfraction the said electrons in divergent direcntions -:p-roportionalto the respective velocities thereo'f', electron barrier means providedwith an opening positioned in the path of the said disapfirse'delectrons to pass a monokinetio beam of electrons'of substantially likevelocity, said barrier means'and said generating means beingelectrica-lly interconnected and enclosing the said ldiffract'ionelement in a substantially equipotenzm'tial space; said accelerating anddirecting means to vary the angle'of incidence of said generated':electron beam on said 'difiracting surface in aci-":- :cordan'ce'=withelectrical control energy, a target 45. electrode to collect the saidpassed electrons, and 1 means intermediate said opening and said targetelectrode to'rnodulate said passed electron beam. 8. A thermionicdis-charge tube comprising an evacuated'envelope, and within saidenvelope an electron diffraction'element, an electron gun pro vided withan electron lens system to produce a beam of electrons directed towardsaid diffraction element; said element having a lattice structurearranged to disperse the said electrons in diver- 5-51 gent directionsproportional to the respective velocities -thereof, electron barriermeans provided .with'an opening positioned in the path of the saiddispersed electrons to pass a monokinetic ."uAbearr'r-of electrons ofsubstantially like velocity, 60."; said-barrier means and said electrongun means being-electrically interconnected and enclosing 2;. the said'difiraction element in a substantially aliequipotentlal space; the sumof the mean paths 4. of the incident electron beam and the diffracted'65 electron bearnto said opening being substantially equal-'to thefocal distance'of said electron lens system',- a target electrode tocollect the said passed electron beam, and means intermediateisaid-opening and said target electrode to modula'te said passed"electron beam.

9. A'thermionic discharge tube comprising an H evacuated envelop-e, andWithin the envelope an electron-diffraction element having a surface thecross se'ction of which'is a concave substantially circular'linefimeans'for generating a beam of i evacuatedenvelope and within theenvelope adif -fraction element, means for generating a beam of tice'structurearranged-to disperse by difiraction i-ncludin'g 'm'agnetic field beamdeflecting means electrons, an electron lens system for focusing saidbeam of electrons and directing said beam toward said surface, saidelement having a substantially regular lattice structure at said surfaceelastically curved in the plane of said cross-section with a radius ofcurvature substantially equal to the diameter of the circle of saiddiffracting line, said beam being diffracted by said element to dispersethe said electrons with respective velocities approximately as afunction of the glancing angles of the incident and diffracted beams,electron barrier means provided with an opening positioned in the pathof the said diffracted electrons to pass a monokinetic beam of electronsof substantially like velocity, said barrier means and said lens systembeing electrically interconnected and enclosing the said diffractionelement in a substantially equipotential space, the sum of the meanpaths of said incident beam and the diffracted beam to said openingbeing substantially equal to the focal distance of the said electronlens system, the said circle passing substantially through the source ofsaid incident beam and through said opening, means to vary the velocityof the electrons of said passed beam in accordance with electricalcontrol energy, a target electrode to collect the said passed electronbeam, and means intermediate said opening and said target electrode tomodulate said passed electron beam.

10. A thermionic discharge tube comprising an evacuated envelope, andwithin the envelope an electron diffraction element having a surface thecross-section of which is a concave substantially circular line, meansfor generating a beam of electrons, an electron lens system for focusingsaid beam of electrons and directin said beam toward said surface, saidelement having a substantially regular lattice structure at said surfaceelastically curved in the plane of said cross-section with a radius ofcurvature substantially equal to the diameter of the circle of saiddiffracting line, said beam being diffracted by said element to dispersethe said electrons approximately as a function of the respectivevelocities thereof, electron barrier means provided with an openingpositioned in the path of the said diffracted electrons to pass amonokinetic beam of electrons of substantially like velocity, saidbarrier means and said electron gun means being electricallyinterconnected and enclosing the said diffraction element in asubstantially equipotential space, the sum of the mean paths of saidincident beam and the diffracted beam to said opening beingsubstantially equal to the focal distance of the said electron lenssystem, the said circle passing substantially through the source of saidgenerated beam and through said opening, said electron lens system beingadapted to vary the velocity of the electrons of the passed beam inaccordance with electrical control energy, a target electrode to collectthe said passed electron beam, and means intermediate said targetelectrode and said opening to modulate said passed electron beam.

11. A thermionic discharge tube comprising an evacuated envelope, andwithin the envelope an electron diffraction element having a surface thecross-section of which is a concave substantially circular line, meansfor generating a beam of electrons, an electron lens system for focusingsaid beam of electrons and directing said beam toward said surface, saidelement having a substantially regular lattice structure at said surfaceelastically curved in the plane of said crosssection with a radius ofcurvature substantially equal to the diameter of the circle of saiddiffracting line, said beam being diffracted by said element to dispersethe said electrons with respective velocities approximately as afunction of the glancing angles of the incident and diffracted beams,electron barrier means provided with an opening positioned in the pathof the said diffracted electrons to pass a monokinetic beam of electronsof substantially like velocity, said barrier means and said electron gunmeans being electrically interconnected and enclosing the saiddiffraction element in a substantially equipotential space, the sum ofthe mean paths of said focused beam from said lens system to saidelement and the diffracted beam from said element to said opening beingsubstantially equal to the focal distance of the said electron lenssystem, the said lens system being adapted to vary the angle ofincidence of said focused beam on said element in accordance with anelectrical control quantity, the said circle passing substantiallythrough the source of said focused beam and through said opening, atarget electrode to collect the said passed electron beam, and meansintermediate said opening and said target electrode to modulate saidpassed electron beam.

ANDRES LEVIALDI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,126,286 Schlesinger Aug. 9,1938 2,158,314 Von Korshenewsky May 16, 1939 2,197,033 Diels Apr. 16,1940 2,260,041 Mahl et al Oct. 21, 1941 2,281,325 Ramo Apr. 28, 19422,372,422 Hillier Mar. 27, 1945 OTHER REFERENCES Davisson and Germer:Diffraction of Electrons by a Crystal of Nickel, page 4, Bell TelephoneLaboratories Reprint B-281, January 1928.

Germer: Optical Experiments with Electrons, page 4, Bell TelephoneLaboratories Reprint B-351, October 1928. (Copies of both publicationsin Div. 54.)

